How to efficiently collect nftables meter values?

[Piotr Jurkiewicz <piotr.jerzy.jurkiewicz@xxxxxxxxx>]

Hi,

I am looking for a way to efficiently collect meter values (several 
times per second).

1. So far the only way I found is to use nft command line binary or 
libnftables. But I do not think it is particularly efficient, since it 
requires to re-exec nft binary/recreate of nft_ctx for every call.

When nft is used in interactive mode or the same ntf_ctx is reused, 
meter values are not updated between subsequent calls:

nft> list meter filter meter_in
table ip filter {
        meter meter_in {
                type inet_proto . ipv4_addr
                size 65535
                flags dynamic
                elements = { udp . 10.127.0.77 counter packets 107631 
bytes 104115478, udp . 10.127.0.81 counter packets 111136 bytes 
107497046, udp . 10.127.0.85 counter packets 207759 bytes 200961171, udp 
. 10.127.0.89 counter packets 165015 bytes 159615602 }
        }
}
nft> list meter filter meter_in
table ip filter {
        meter meter_in {
                type inet_proto . ipv4_addr
                size 65535
                flags dynamic
                elements = { udp . 10.127.0.77 counter packets 107631 
bytes 104115478, udp . 10.127.0.81 counter packets 111136 bytes 
107497046, udp . 10.127.0.85 counter packets 207759 bytes 200961171, udp 
. 10.127.0.89 counter packets 165015 bytes 159615602 }
        }
}

I believe this is a bug.

2. Is there any userspace collector similar to ulogd2 to collect meter 
values?

3. I can write a simple collector myself. But I need nftables Netlink 
API documentation, similar to 
http://man7.org/linux/man-pages/man7/rtnetlink.7.html

Is there any NFNL_SUBSYS_NFTABLES documentation?

Best regards,
Piotr